CE PoC, trying out custom operations in a CE that has `bind` and `yield`, which seems impossible yet

[abelbraaksma]

So, we can add CustomOperation operations to a CE that has bind (but the requirements are rather odd), or we can have them with yield (similar to how query works in F# Core), but the combination of the two seems to be impossible.

Since TaskSeq has both bind (unpack a task or async) and yield (act like a sequence), this is a problem (well, to the extend that we'd want custom operations, but before I decide I like to know how to do it to begin with).

Nothing of this is currently slated to go into main, it may be that there is no good way to fix this until F# itself has this (assuming it is a bug).